Residual Gain Monitoring and Reduction for EUV Drive Laser

ABSTRACT

A laser system includes a laser source operable to provide a laser beam; a laser amplifier having an input port and an output port and operable to amplify the laser beam, the laser beam travelling along a main beam path through the laser amplifier from the input port to the output port; and a residual gain monitor operable to provide a probe laser beam, the probe laser beam travelling along a probe beam path through the laser amplifier from the output port to the input port, wherein the residual gain monitor calculates a residual gain of the laser amplifier according to the probe laser beam.

PRIORITY

This application claims the benefits of U.S. Prov. App. No. 62/491,806entitled “Residual Gain Monitoring and Reduction for Laser System,”filed Apr. 28, 2017, herein incorporated by reference in its entirety.

BACKGROUND

The electronics industry has experienced an ever increasing demand forsmaller and faster electronic devices which are simultaneously able tosupport a greater number of increasingly complex and sophisticatedfunctions. Accordingly, there is a continuing trend in the semiconductorindustry to manufacture low-cost, high-performance, and low-powerintegrated circuits (ICs). Thus far these goals have been achieved inlarge part by scaling down semiconductor IC dimensions (e.g., minimumfeature size) and thereby improving production efficiency and loweringassociated costs. However, such scaling has also introduced increasedcomplexity to the semiconductor manufacturing process. Thus, therealization of continued advances in semiconductor ICs and devices callsfor similar advances in semiconductor manufacturing processes andtechnology.

For example, semiconductor lithography processes may use lithographictemplates (e.g., photomasks or reticles) to optically transfer patternsonto a substrate. Such a process may be accomplished by projection of aradiation source, through an intervening photomask or reticle, onto thesubstrate having a photosensitive material (e.g., photoresist) coating.The minimum feature size that may be patterned by way of such alithography process is limited by the wavelength of the projectedradiation source. In view of this, extreme ultraviolet (EUV) lightsources and lithographic processes have been introduced. In addition,EUV lithographic processes may save manufacturing cost by avoiding aneed to apply a multi-patterning technique in achieving minimum featuresizes.

However, generating the EUV light (or radiation) in EUV light generationsystems can be an energy intensive and difficult process to control. Asmerely one example, a method to produce EUV light includes utilizing alaser system to generate a laser beam to irradiate a material that inturn radiates EUV light. After amplifying the laser beam, the lasersystem may still have residual energy left in its gain medium. Suchresidual energy can be harmful to the laser system when a portion of thelaser beam is reflected along the laser beam path and travels back intothe gain medium. The reflected laser beam in backward direction receivesresidual gain from the gain medium and gets amplified. The amplifiedreflected laser beam may generate extra heat that requires dissipation,or may become too strong in energy level and cause damages to opticalcomponents in the laser system. Moreover, the residual gain may induceself-lasing effect in amplifier chain to affect the temporal domainperformance of laser pulses in forward direction, which may furtheraffect a target material formation when laser pulses impinge on suchtarget material, and in turn deteriorate the EUV generation. As such,there is a great deal of interests in tools and techniques capable ofaccurately monitoring residual gain and/or reducing residual gain in thelaser system.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic view of an EUV light (also referred to as EUVradiation) source system, including an exemplary laser system and anexemplary EUV vessel, in accordance with some embodiments.

FIGS. 2A, 2B, and 2C are exemplary diagrammatic views of an EUV lightsource system including a laser beam impacting a droplet and generationof EUV light and a reflected laser beam therefrom, in accordance withsome embodiments.

FIGS. 3A, 3B, and 3C illustrate a laser system with metrology apparatusfor residual gain monitoring, in accordance with some embodiments.

FIGS. 4A, 4B, 4C, 4D, 4E, and 4F illustrate a laser amplifier withmulti-pass amplifier configuration, in accordance with some embodiments.

FIG. 5 is a schematic view of a lithography system, in accordance withsome embodiments.

FIG. 6 is a flow chart of a method for performing residual gainmonitoring, in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. Additionally, throughoutthe present disclosure, the terms “mask”, “photomask”, and “reticle” maybe used interchangeably to refer to a lithographic template, such as anEUV mask.

As the minimum feature size of semiconductor integrated circuits (ICs)has continued to shrink, there has continued to be a great interest inphotolithography systems and processes using radiation sources withshorter wavelengths. In view of this, extreme ultraviolet (EUV) lightsources, processes, and systems have been introduced. In addition, EUVlithographic processes may save manufacturing cost by avoiding a need toapply a multi-patterning technique in achieving minimum feature sizes.Methods to produce EUV light include, but are not necessarily limitedto, converting a material into a plasma state that has an element (e.g.,xenon, lithium, or tin) with an emission line in the EUV spectrum. Inone such method, often termed laser produced plasma (LPP), the requiredEUV light can be produced by irradiating a target material, for examplein the form of a droplet, with a laser beam emitted from a laser system.In accordance with its various embodiments, the present disclosure isgenerally related to metrology tools and techniques capable ofmonitoring residual gain in the laser system and methods of reducingsuch residual gain.

Referring to FIG. 1, illustrated therein is a schematic view of a EUVlight generation system 100. The EUV light generation system 100 isillustrative of an exemplary system that creates EUV wavelengthradiation, which can be delivered to a EUV lithography system 110, whichwill be further described later in FIG. 5. In some embodiments, a EUVlight generation system 100 may include a laser produced plasma (LPP)EUV light source. Thus, as shown and in some embodiments, the EUV lightgeneration system 100 may include a laser system 112 for generating anddelivering a laser beam 114 to a EUV vessel 116.

The laser beam 114 may be a continuous beam or a series of pulses. Insome embodiments, the laser beam 114 includes one or more main pulses,and/or one or more pre-pulses. Suitable lasers generated by the lasersystem 112 may include KrF, ArF, CO₂ lasers, solid-state laser, andother appropriate lasers. As an example, the laser system 112 mayinclude a pulse laser device (e.g., a pulsed gas-discharge CO₂ laserdevice) producing a laser radiation at 9.2 um or 10.6 um, with DC or RFexcitation, operating at a relatively high power (e.g., 20 KW or higher)and a high pulse repetition rate (e.g., 50 KHz or more).

In the illustrated embodiment, the laser system 112 has a masteroscillator power amplifier (MOPA) configuration, which includes a masteroscillator (MO) 118 as a seed laser source and multiple stages of poweramplifiers (PA) 120. MOPA configuration can be used to not only optimizethe amplification of a range of input parameters (e.g. laser pulsewidth, wavelength bandwidth etc.), but also amplify the input signalwith both high gain and high efficiency in a laser system, as well as tocontrol the input signal and its amplification in series separately,such that both the required input signal parameter and its amplificationcan be optimized. Using a master oscillator 118, for example, the highquality laser beam 114 with a pulse width short to 30 ns may begenerated for extremely high-peak intensity at low pulse energy to driveEUV with higher conversion efficiency. Using power amplifiers 120 in achain configuration, for example, the laser beam 114 can be furtherintensified efficiently, in order to deliver the power levels necessaryfor the high throughput. The master oscillator 118 is also referred toas the seed laser source 118. In a particular embodiment, the seed lasersource 118 is a Q-switched or mode-locking laser source. The poweramplifier 120 is also referred to as the laser amplifier 120. In aparticular embodiment, the laser amplifier 120 is a RF pumped, fastaxial flow, CO₂ laser amplifier.

In the EUV light generation system 100, the laser beam 114 may then bedirected, by a beam transport and focus system 122, to the EUV vessel116. The path along which the laser beam 114 travels through from theseed laser source 118 into the EUV vessel 116 is defined as the laserbeam path. The chamber of beam path represented by the box 122 of FIG. 1may include various devices to perform various functions including beamtransport, beam focusing, beam amplification, and/or other suitablefunctionality.

In various embodiments, the EUV vessel 116 also includes a targetgenerator 124 and a target catcher 126. In some cases, the targetgenerator 124 is a droplet generator which provides target 128 in a formof droplets (such as tin or a tin compound, discussed further below)into the EUV vessel 116.

The EUV vessel 116 may include one or more optical elements such as acollector 130. In some embodiments, the collector 130 may include anormal incidence reflector, for example, implemented as a multilayermirror (MLM). For example, the collector 130 may include a siliconcarbide (SiC) substrate coated with a Mo/Si multilayer. In some cases,one or more barrier layers may be formed at each interface of the MLM,for example, to block thermally-induced interlayer diffusion. In someexamples, other substrate materials may be used for the collector 130such as Al, Si, or other type of substrate materials. The collector 130may be an ellipsoid-shape with an aperture (or opening) 132 at thecenter to allow the laser beam 114 to pass through and reach anirradiation region 134. Thus, in some embodiments, the laser beam 114passes through the aperture 132 of the collector 130 and irradiatesdroplets 128 generated by the droplet generator 124, thereby producingplasma at the irradiation region 134. In some embodiments, the collector130 may have a first focus at the irradiation region 134 and a secondfocus at an intermediate focus region 136. By way of example, the plasmagenerated at the irradiation region 134 produces EUV light 138 collectedby the collector 130 and output from the EUV vessel 116 through theintermediate focus region 136. From there, the EUV light 138 may betransmitted to an EUV lithography system 110 for processing of asemiconductor substrate. The generated EUV light 138 is anelectromagnetic radiation having wavelengths of around 50 nm or less(also sometimes referred to as soft x-rays). In an embodiment, the EUVlight 138 includes a wavelength centered around about 13.5 nm.

The interaction between the laser beam 114 and the target 128 isdescribed in greater detail below with reference to FIGS. 2A and 2B.Referring to FIG. 2A, illustrated are diagrammatic view of portions ofthe EUV vessel 116, which provide further details that may be applied tothe system of FIG. 1. FIG. 2A shows a diagrammatic view including thecollector 130 and an entry of the laser beam 114 through the collectoraperture 132 and incident upon a target 128 at the irradiation region134. The target 128 may be the liquid droplet in spherical shape or theexpanded mist in ellipsoidal shape. The material of the target 128 mayinclude xenon, lithium, tin, indium, antimony, or tellurium, with anemission line in the EUV spectrum. In one embodiment, the target 128 mayinclude tin or a tin compound. Example compositions include, but are notlimited to, tin, SnBr4, SnBr2, SnH4, tin-gallium alloys, tin-indiumalloys, tin-indium-gallium alloys or combinations thereof. The target128 may have a diameter of approximately 10 um to 150 um. Through theamplification from multiple laser amplifiers 120, the laser beam 114 mayhave a power level ranging from about 10 KW to 40 KW, such as 26 KW inan example, before irradiating the target 128. Upon the irradiation, thematerial in the target 128 is converted by the laser beam 114 into aplasma state and emits EUV light, illustrated as the EUV light 138. Itis noted that in an exemplary embodiment, the collector 130 may beapproximately 24 inches in diameter with a 4 inch diameter aperture 132in the center. The EUV light 138 may be angularly distributed such thatit is incident upon the mirror surface of the collector 130. The EUVlight 138 is further collected and focused by the collector 130 to afocal point, such as the focal region 136 in FIG. 1.

FIG. 2B illustrates that a portion of the laser beam 114 is alsoreflected back from the target 128, illustrated as the reflected laserbeam 114′. The reflection in FIG. 2B may happen contemporaneously as theemitting of the EUV light 138 in FIG. 2A. The reflected laser beam 114′may travel along the laser beam path of the laser beam 114, but in anopposite direction. It may trace back to the laser system 112 throughthe aperture 132 and the beam transport and focus system 122 in FIG. 1.The residual energy remains in the laser amplifiers 120 may furtheramplify the reflected laser beam 114′. The gain (or referred as theamount of amplification) received by the reflected laser beam 114′ fromthe laser amplifiers 120 is termed residual gain. In some embodiments,the residual gain may boost the amplified reflected laser beam 114′ to apower level above 1 KW back to the master oscillator of the laser system112, which can create thermal dissipation and/or stabilization issues.In various embodiments, there is a need to monitor and reduce theresidual gain in the laser amplifiers 120 to avoid damages in the EUVlight system and thereby free from EUV source downtime during aproduction, and also to maintain the target formation. Moreover, if theresidual gain 120 goes beyond the specific lasing threshold in the laseramplifiers, a self-lasing may occur and contribute to the temporal noiseat front foot of main pulse. This noise affects the target 128 formationright before the main pulse impinging on the target (e.g., a target 128in irregular shapes), as shown on FIG. 2C.

It is noted that in FIGS. 2A and 2B, the target 128 is elliptical (alsoreferred as a “pancake” shape) in the cross-sectional view. In otherembodiments, the target 128 may be approximately spherical. Theelliptical shape may be provided by introducing a pre-pulse of the laser(e.g., a CO₂ laser) from the seed laser source 118 prior to theintroduction of a main-pulse of the laser beam 114. The pre-pulse may beused to shape the target 128 increasing the available surface area forimpact with a subsequent main pulse of the laser beam 114.

A laser system with metrology tool for monitoring residual gain of itslaser amplifiers is described in detail below with reference to FIGS.3A, 3B, and 3C. Referring to FIG. 3A, illustrated is a schematic view ofthe laser system 112, which provides further details of the EUV lightgeneration system 100 of FIG. 1. The laser system 112 includes the seedlaser source 118 and one or more laser amplifiers 120 as discussed abovein FIG. 1. In the illustrated embodiment, two laser amplifiers 120 areprovided for illustration purposes and do not necessarily limit theembodiments of the present disclosure to any number of laser amplifiers120. Further, although not shown, it should be recognized by one ofordinary skill in the art that various other elements can be included ina laser system, such as optical components and electronic controlcircuits.

Each laser amplifier 120 includes a gain medium 150. The laser beam 114enters the gain medium 150 from an input port 152 of the laser amplifier120 and exits from an output port 154. The gain medium 150 can amplifythe power of (i.e., provide gain to) a laser beam passing through it. Inorder to amplify a laser beam, the gain medium must be in a nonthermalenergy distribution known as a population inversion. The preparation ofthis state requires an external energy source and is known as laserpumping. The gain results from the stimulated emission of gain mediumcomposition's transitions to a lower energy state from a higher energystate previously populated by the laser pumping. Depending on the typesof the laser amplifier, compositions of the gain medium 150 may includecrystals doped with rare-earth ions or transition metal ions, silicateor phosphate glasses, mixtures of gases, semiconductors, or liquids inthe form of dye solutions.

In the illustrated embodiment, the laser amplifier 120 is a CO₂ laseramplifier based on a gas mixture as the gain medium 150, which maycontain CO₂, He, N₂, and possibly some H₂, water vapor, and/or Xe. A CO₂laser amplifier can be electrically pumped via a gas discharge, whichcan be operated with DC current, AC current (e.g., 20-50 kHz), or radiofrequency (RF) energy. N₂ molecules are excited by the discharge into ametastable vibrational level and transfer their excitation energy to theCO₂ molecules when colliding with them. He molecules serve to depopulatethe lower laser level and to remove the heat. Other constituents such asH₂ or water vapor can help (particularly in sealed-tube laseramplifiers) to reoxidize carbon monoxide (formed in the discharge) tocarbon dioxide. In one embodiment, the gas mixture has a ratio ofCO₂:He:N₂ about 5%:80%:15%.

The laser beam 114 has a beam width w₁. The laser beam 114 along thebeam path may not have sharp edges, but usually has certain distributionin a transverse plane, such as a Gaussian distribution. The beam widthw₁ of the laser beam 114 is herein defined as the distance between twopositions in a plane perpendicular to the beam axis where the laserintensity drops to a pre-defined level of the value on the beam axis,such as when a marginal distribution drops to 1/e² (about 13.5%) timesthe maximum value. The beam path of the laser beam 114 is alsoconsidered to have the same width w₁ with the laser beam 114. The valueof the beam width w₁ may change along the beam path, for example, whenthe laser beam 114 passes some optical components and causes thedistribution in the transverse plane changed.

The gain in the gain medium 150 is consumed (or attenuated) after energyhas been provided to amplify the laser beam 114. However, there may besubstantial energy remains in the gain medium 150, as residual energy.When the reflected laser beam 114′ (reflected from the EUV vessel 116and/or other portions of the laser beam 114 reflected at interfacesbetween various optical components along the beam path), travels backinto the gain medium 150, the residual energy provides residual gain tothe reflected laser beam 114′. To monitor the residual gain, the lasersystem 112 further has one or more residual gain monitors 160 coupled tothe laser amplifiers 120.

Still referring to FIG. 3A, in the illustrated embodiment, each laseramplifier 120 is coupled to a residual gain monitor 160. The residualgain monitor 160 has an emitter 162 and a receiver 164. A probe laserbeam 166 is transmitted from the emitter 162, which enters the laseramplifier 120 from the output port 154 and exits from the input port152, and received by the receiver 164. Along the beam path of the probelaser beam 166, there may be several reflectors 168 that help guide theprobe laser beam 166 into the laser amplifier 120. For the sake ofclarity, the beam path of the laser beam 114 is referred to as the mainbeam path, and the beam path of the probe laser beam 166 is referred toas the probe beam path. The probe beam path has a width w₂ associatedwith a distribution of the probe laser beam 166 in a transverse plane.The probe beam path may be parallel to (FIG. 3A) or diagonally cross(FIG. 3B) the main beam path inside the respective laser amplifier 120.The probe laser beam 166 may be generated by continuous or pulsedlasers.

Both pulse energy and average power of the probe laser beam 166 isselected at the level far less than amplifier saturation energy andpower, respectively, such that the probe laser beam 166 would notsubstantially change the residual gain in the gain medium 150, evenafter being amplified. In some embodiments, the probe laser beam 166 hasa power level ranging from about 1 uW to about 1 mW and an energy levelranging from about 1 nJ to about 1 uJ before entering the respectivelaser amplifier 120. In some embodiments, the probe laser beam 166 has apower level ranging from has a power level ranging from 10⁻⁹ to 10⁻⁶ ofa power level of the laser beam 114 and an energy level ranging from10⁻⁶ to 10⁻³ before both entering the respective laser amplifier 120.After traveling through the laser amplifier 120, the probe laser beam166 is amplified by the residual energy that remains in the gain medium150. By comparing the amount of amplification, the residual gain monitor160 measures a residual gain of the probe laser beam 166. The residualenergy in the gain medium 150 may have a distribution. The residual gainof the probe laser beam 166 may not necessarily be the same as theresidual gain of the reflected laser beam 114′. For example, if theprobe beam path is offset from the main beam path in the gain medium 150as shown in FIG. 3A. By associating a known or estimated distribution ofthe residual energy and the measured residual gain of the probe laserbeam 166, the residual gain monitor 160 may calculate (or predict) theresidual gain of the reflective laser beam 114′ (also referred to as theresidual gain of the laser amplifier).

Each residual gain monitor 160 is coupled to a control module 170through a communication line 172. The communication line 172 may be adirect wire connection, or include a bus, such as an Inter-integratedCircuit (I²C) bus, a Serial Peripheral Interface (SPI) bus, or aUniversal Asynchronous Receiver Transmitter (UART) bus. The controlmodule 170 retrieves residual gain information from the residual gainmonitors 160 and may compare it to a threshold level. The control module170 is further coupled to the laser system 112 through a control signalline 174. If residual gain from one of the laser amplifiers 120 is toohigh, the control module 170 can adjust parameters of the respectivegain medium 150 to lower the residual gain, such as by reducing thestrength of the laser pumping or missing target to terminate EUVgeneration by shifting the time delay between laser pulse and movingtarget.

As illustrated in FIG. 3A, the probe beam path may not overlap with themain beam path in the gain medium 150. Therefore, the laser beam 114(and/or the reflected laser beam 114′) and the probe laser beam 166 maypropagate inside the laser amplifier 120 simultaneously withoutinterfering with each other, which allows the residual gain to bemonitored on the fly.

Referring to FIG. 3B, the probe beam path may overlap with the main beampath in the gain medium 150 in some embodiments. In furtherance of theseembodiments, the probe beam path is overlapped with the main beam paththrough diagonally crossing by just rotating the input mirror of probebeam. In another embodiment, the reflectors 168 are reconfigurable,allowing the probe beam path to be either offset from the main beam pathor diagonally crossing the main beam path in one apparatus, before orduring transmitting the main beam. In yet another embodiment, the probebeam path and the main beam path are co-axial. The probe laser beam 166is transmitted into the gain medium 150 during a time period when thereare no other laser beams inside the gain medium 150. For example, thelaser beam 114 being a series of laser pulses, after a laser pulse exitsthe gain medium 150, and before a reflected laser pulse 114′ enters thegain medium 150, the probe laser beam 166 is transmitted as a probelaser pulse 166 into and travels through the gain medium 150. Since theprobe beam path overlaps with the main beam path, the measured residualgain of the probe laser beam 166 may be considered as the residual gainof the laser amplifier 120.

In various embodiments, the number of residual gain monitors 160 may beless than the number of laser amplifiers 120. In some embodiments, thelaser amplifier 120 positioned in the last stage along the main beampath provides the largest power increment to the laser beam 114 but withthe smallest gain (also known as a power stage), while the first fewlaser amplifiers 120 closer to the seed laser source 118 provide smallerpower increment to the laser beam 114 but much larger gain (also knownas pre-amplify stages). In such a scenario, the laser amplifier 120positioned in the last stage may not need a residual gain monitor 160for residual gain monitoring, as the residual gain at this stage may bemuch lower than a threshold level. In some embodiments, the laser system112 may have just one laser monitor 120 to monitor the residual gain ofthe first laser amplifier 120 positioned next to the seed laser source118, which may have the largest residual gain among the laser amplifiers120. In alternative embodiments, the laser system 112 may have just oneresidual gain monitor 120, while the residual gain monitor emits theprobe laser beam 166 into the output port 154 of the last laseramplifier 120 positioned in the main beam path and collects the probelaser beam 166 from the input port 152 of the first laser amplifier,thereby monitoring the residual gain of all the laser amplifiers 120 asa whole.

FIG. 3C illustrates some embodiments of the laser system 112 withmultiple laser amplifiers 120 in different gain medium sizes.Specifically, cross-sectional areas of the gain medium 150 of multiplelaser amplifiers 120 are enlarged gradually along the propagationdirection of the laser beam 114. Meanwhile, the beam width of the laserbeam 114 is also enlarged when traveling from one gain medium intoanother (e.g., from w₁ to w₁′ as illustrated in FIG. 3C) for higheramplifier saturation energy stage by stage. The beam width enlargmentcan be implemented by inserting a beam-shaping optics 180 (e.g., atelescopic beam expander) between two laser amplifiers 120. In someembodiments, the gain medium 150 is in a tube shape, and the ratio ofthe beam width enlargement w₁′/w₁ is the same as the ratio of the radiusenlargement of the respective two gain mediums 150. In some embodiments,the beam width of the laser beam 114 is tuned to match the radius of thegain medium 150 in each stage, leaving smaller areas outside of the mainbeam path in a gain medium. Since energy stored in areas outside of themain beam path in a gain medium may contribute to a major portion of theresidual energy, by matching the main beam width with thecross-sectional size of the gain medium, areas outside of the main beampath in a gain medium are reduced. In turn, less residual energy remainsin those areas and the residual gain in the gain medium is therebyreduced. In a specific example, the laser gas tube size of the firstlaser amplifier 120 (pre-amplify stage) has a diameter of about 25 mm,and the ratio of the tube size along a 4-stage MOPA is about1:1:1.3:1.5. Furthermore, the output beam size may have a diameter ofabout 20 mm, and the ratio of beam size along the 4-stage MOPA is about1.3:1:1.13:1. Optionally, each laser amplifier 120 may also include aresidual gain monitor 160 for residual gain monitoring. The probe beampath may overlap with (e.g., diagonally overlap as illustrated in FIG.3B) or be co-axial with the main beam path, or be offset from the mainbeam path (e.g., offset as illustrated in FIG. 3A and FIG. 3C). Inanother embodiment, the probe beam path in the first laser amplifier 120is offset from the main beam path and the probe beam path in the secondlaser amplifier 120 diagonally overlaps with the main beam path, or viceversa, for example, to accommodate the placement of reflectors 168 onboth sides of the main beam.

A laser system with a multi-pass amplifier configuration capable ofreducing residual gain is described in detail below with reference toFIGS. 4A-4F. The laser amplifier illustrated in FIGS. 4A-4F is similarto the ones in FIG. 1 and FIGS. 3A-3C in various aspects. Therefore,reference numerals are repeated to show the same or similar componentsin the illustrated embodiments. Furthermore, some descriptions of thesame or similar components in FIGS. 4A-4F are abbreviated or omitted byreferring to the descriptions above in FIG. 1 and FIGS. 3A-3C for thesake of simplicity.

FIG. 4A illustrates an exemplary laser amplifier 120 with a multi-passamplifier configuration. In a multi-pass amplifier configuration, alaser beam output from a laser amplifier makes at least two separatepasses through the gain medium of the laser amplifier. The use of atleast one additional pass can allow for an increase in gain extraction,and can provide the ability to obtain higher output laser beam energywith a lower input energy requirement with the same RF pumping to driveEUV generation. Such a configuration can also be used to drive the laseramplifier into a state of saturation, thereby reducing pulse-to-pulseenergy fluctuations and improving beam homogeneity, in spite of reducingthe number of laser amplifier along MOPA. Furthermore, the at least oneadditional pass can bring away additional energy from the gain medium,resulting in less residual energy remaining after the laser beam haspassed through, and thereby reducing residual gain of the laseramplifier.

In the illustrated embodiment, the laser amplifier 120 has apolarization beam splitter 410 in front of the input port 152 and aretro-reflector 412 after the output port 154. The polarization beamsplitter 410 allows a laser beam's polarization components in adirection (e.g., in a horizontal direction) to pass through, whilereflects other polarization components perpendicular to that direction(e.g., in a vertical direction). In the illustrated embodiment, thelaser beam 114 has horizontal polarization, which allows it to pass thepolarization beam splitter 410 and travel through the gain medium 150 ina first pass until reaching the retro-reflector 412. The retro-reflector412 reflects the laser beam 114 back to the gain medium 150. Theretro-reflector 412 further changes the polarization of the laser beam114 by 90°, for example, by using a quarter wave plate. After thereflection, the laser beam 114 travels back into the gain medium in asecond pass with a vertical polarization until reaching the polarizationbeam splitter 410 from another side. Since the laser beam 114 in thefirst pass and the laser beam 114 in the second passes haveperpendicular polarizations, they do not interfere with each other.Subsequently, the polarization beam splitter 410 reflects the laser beam114 in the vertical polarization to the next laser amplifier 120.

Since the laser beam 114 passes the laser amplifier 112 twice, the laseramplifier 120 in such a configuration is also referred to as a dual-passlaser amplifier 120. In some embodiments, the laser amplifier 120 mayinclude extra sets of polarization beam splitter and retro-reflector toallow the laser beam to pass the gain medium more than twice.

The metrology apparatus for residual gain monitoring can also beimplemented together with the multi-pass amplifier configuration in thelaser amplifier 112, as illustrated in FIG. 4B. The residual gainmonitor 160 transmits and receives the probe laser beam 166 and measuresthe amount of amplification of the probe laser beam 166. In FIG. 4B, theprobe laser beam 166 is offset from the laser beam 114 and passes thegain medium 150 only once. Therefore, the residual gain monitor 160 needto calculate the residual gain of a multi-pass configuration furtherbased on the gain distribution and other characteristics of the gainmedium 150. As illustrated in FIG. 3B, the probe beam may diagonallycross the laser beam 114 for dual-pass laser amplifier in FIG. 4B.

Referring to FIG. 4C, in yet another embodiment, the laser amplifier 120has a separate set of polarization beam splitter 410′ andretro-reflector 412′ for the probe laser beam 166. The probe laser beam166 has the same polarization direction as the polarization beamsplitter 410′, allowing it to pass the polarization beam splitter 410′toward the gain medium 150 for the first pass. The probe laser beam 166enters the gain medium 150 from the output port 154 and exits from theinput port 152. After being reflected by the retro-reflector 412′, theprobe laser beam 166 travels back to the gain medium 150 for the secondpass with its polarization direction changed by 90°. The polarizationbeam splitter 410′ subsequently reflects the probe laser beam 166 backto the residual gain monitor 160. In FIG. 4C, the probe laser beam 166also has a multiple-pass as the laser beam 114, but offset from thelaser beam 114. Therefore, the residual gain monitor 160 need tocalculate the residual gain of a multi-pass configuration further basedon the gain distribution of the gain medium 150.

Referring to FIG. 4D, the laser beam 114 and the probe beam 166 mayshare the polarization beam splitter 410 and the retro-reflector 412 insome embodiments. Such configuration allows the number of opticalcomponents used in the laser amplifier 120 to be reduced. The main beampath and the probe beam path may overlap or be co-axial in theillustrated embodiment. Therefore, the amount of amplification of theprobe beam 166 can be approximately regarded as the residual gain of thelaser amplifier 120 in a multi-pass amplifier configuration. If theprobe beam is in a pulse train, the time delay between the probe beamand main beam can be scanned in a range of time period back and forth,for example, by using a probe beam delay line 183 to send the probe beambefore or after the main beam at a tunable time interval. Therefore, thetemporal evolution of the amplifier gain depletion can be furtherresolved through tomography to finely optimize the laser dynamics. Forexample, by adjusting the time delay between the probe beam and mainbeam, a curve of the amplifier gain depletion's temporal evolution canbe plotted, showing different working phases of the laser amplifiers,including RF pumping phase, pump saturation phase, laser beam input andamplification phase, residual gain phase, and/or other possible phases.This curve can help fine tuning or optimizing the laser dynamics in thesystem.

Referring to FIG. 4E, an implementation to prevent the reflected laserbeam 114′ from entering the laser amplifier 120 is illustrated. Apolarizing element 420 is placed in the beam path of the reflected laserbeam 114′ before it reaches the polarization beam splitter 410. Thepolarizing element 420 converts the polarization of the reflected laserbeam 114′ to the same polarization direction as the polarization beamsplitter 410 (e.g., in a horizontal direction). This allows thepolarization beam splitter 410 to pass the reflected laser beam 114′without reflecting it into the gain medium 150 from the input port 152.Thus, the reflected laser beam 114′ avoids amplification by the residualgain of the laser amplifier 120. The reflection beam after passingthrough the polarization beam splitter at the last laser amplifier maybe an alternative diagnostic to purely estimate the laser-targetqualification without interfered by the amplification of residual gain.

In some embodiments, the laser amplifier 120 has more than one gainmedium 150, where the implementation to detour the reflected laser beam114′ as illustrated in FIG. 4E may still be adopted. FIG. 4F illustratesthe laser amplifier 120 with gain medium segments A-G. Each gain mediumsegment may be in a tube shape, placed sequentially in a zigzag fashionfor compactness. Optical components between two gain medium segments(not shown) guide the laser beam 114 to transmit from one gain mediumsegment to the next. Each gain medium segment may have differentdiameters, such as the ones illustrated in FIG. 3C. The laser beam 114travels in the same polarization direction as the polarization beamsplitter 410 and enters the gain medium segments from the input port152. The laser beam 114 is amplified sequentially by the gain mediumsegments A-G in a first pass. A retro-reflector 412 is placed at theoutput port 154 and reflects the laser beam 114 back to the gain mediumsegments for a second pass. The retro-reflector 412 also changes thepolarization of the laser beam 412 by 90°. After the laser beam 412 isamplified sequentially by the gain medium segments G-A in the secondpass and leaves the input port 152, the polarization beam splitter 410reflects the laser beam 412 to the next stage, such as the next laseramplifier or the EUV vessel 116 (FIG. 1). The reflected laser beam 114′from the EUV vessel 116 travels backward in the main beam path. Beforethe reflected laser beam 114′ reaches the polarization beam splitter410, the polarizing element 420 adjust the polarization of the reflectedlaser beam 114′ to be the same as the polarization beam splitter 410.Afterwards, the reflected laser beam 114′ passes through thepolarization beam splitter 410, bypassing the laser amplifier 120without being amplified by the residual gain.

As previously noted, the EUV vessel described above may be used toprovide an EUV light source for a lithography system. By way ofillustration, and with reference to FIG. 5, provided therein is aschematic view of an exemplary lithography system 500, in accordancewith some embodiments. The lithography system 500 may also begenerically referred to as a scanner that is operable to performlithographic processes including exposure with a respective radiationsource and in a particular exposure mode. In at least some of thepresent embodiments, the lithography system 500 includes an extremeultraviolet (EUV) lithography system designed to expose a resist layerby EUV light. In various embodiments, the resist layer includes amaterial sensitive to the EUV light (e.g., an EUV resist). Thelithography system 500 of FIG. 5 includes a plurality of subsystems suchas a radiation source 502, an illuminator 504, a mask stage 506configured to receive a mask 508, projection optics 510, and a substratestage 518 configured to receive a semiconductor substrate 516. A generaldescription of the operation of the lithography system 500 may be givenas follows: EUV light from the radiation source 502 is directed towardthe illuminator 504 (which includes a set of reflective mirrors) andprojected onto the reflective mask 508. A reflected mask image isdirected toward the projection optics 510, which focuses the EUV lightand projects the EUV light onto the semiconductor substrate 516 toexpose an EUV resist layer deposited thereupon. Additionally, in variousexamples, each subsystem of the lithography system 500 may be housed in,and thus operate within, a high-vacuum environment, for example, toreduce atmospheric absorption of EUV light.

In the embodiments described herein, the radiation source 502 may beused to generate the EUV light. As discussed above, the source maygenerate the EUV light using a laser produced plasma (LPP). In someexamples, the EUV light may include light having a wavelength rangingfrom about 1 nm to about 100 nm. In one particular example, theradiation source 502 generates EUV light with a wavelength centered atabout 13.5 nm. Accordingly, the radiation source 502 may also bereferred to as an EUV radiation source 502. In some embodiments, theradiation source 502 also includes a collector, which may be used tocollect EUV light generated from the plasma source and to direct the EUVlight toward imaging optics such as the illuminator 504.

Upon receipt, light from the radiation source 502 is directed toward theilluminator 504. In some embodiments, the illuminator 504 may includereflective optics (e.g., for the EUV lithography system 500), such as asingle mirror or a mirror system having multiple mirrors in order todirect light from the radiation source 502 onto the mask stage 506, andparticularly to the mask 508 secured on the mask stage 506. In someexamples, the illuminator 504 may include a zone plate, for example, toimprove focus of the EUV light. In some embodiments, the illuminator 504may be configured to shape the EUV light passing therethrough inaccordance with a particular pupil shape, and including for example, adipole shape, a quadrapole shape, an annular shape, a single beam shape,a multiple beam shape, and/or a combination thereof. In someembodiments, the illuminator 504 is operable to configure the mirrors(i.e., of the illuminator 504) to provide a desired illumination to themask 508. In one example, the mirrors of the illuminator 504 areconfigurable to reflect EUV light to different illumination positions.In some embodiments, a stage prior to the illuminator 504 mayadditionally include other configurable mirrors that may be used todirect the EUV light to different illumination positions within themirrors of the illuminator 504. In some embodiments, the illuminator 504is configured to provide an on-axis illumination (ONI) to the mask 508.In some embodiments, the illuminator 504 is configured to provide anoff-axis illumination (OAI) to the mask 508. It should be noted that theoptics employed in the EUV lithography system 500, and in particularoptics used for the illuminator 504 and the projection optics 510, mayinclude mirrors having multilayer thin-film coatings known as Braggreflectors. By way of example, such a multilayer thin-film coating mayinclude alternating layers of Mo and Si, which provides for highreflectivity at EUV wavelengths (e.g., about 13 nm).

As discussed above, the lithography system 500 also includes the maskstage 506 configured to secure the mask 508. Since the lithographysystem 500 may be housed in, and thus operate within, a high-vacuumenvironment, the mask stage 506 may include an electrostatic chuck(e-chuck) to secure the mask 508. As with the optics of the EUVlithography system 500, the mask 508 is also reflective. As illustratedin the example of FIG. 5, light is reflected from the mask 508 anddirected towards the projection optics 510, which collects the EUV lightreflected from the mask 508. By way of example, the EUV light collectedby the projection optics 510 (reflected from the mask 508) carries animage of the pattern defined by the mask 508. In various embodiments,the projection optics 510 provides for imaging the pattern of the mask508 onto the semiconductor substrate 516 secured on the substrate stage518 of the lithography system 500. In particular, in variousembodiments, the projection optics 510 focuses the collected EUV lightand projects the EUV light onto the semiconductor substrate 516 toexpose an EUV resist layer deposited on the semiconductor substrate 516.As described above, the projection optics 510 may include reflectiveoptics, as used in EUV lithography systems such as the lithographysystem 500. In some embodiments, the illuminator 504 and the projectionoptics 510 are collectively referred to as an optical module of thelithography system 500.

In some embodiments, the lithography system 500 also includes a pupilphase modulator 512 to modulate an optical phase of the EUV lightdirected from the mask 508, such that the light has a phase distributionalong a projection pupil plane 514. In some embodiments, the pupil phasemodulator 512 includes a mechanism to tune the reflective mirrors of theprojection optics 510 for phase modulation. For example, in someembodiments, the mirrors of the projection optics 510 are configurableto reflect the EUV light through the pupil phase modulator 512, therebymodulating the phase of the light through the projection optics 510. Insome embodiments, the pupil phase modulator 512 utilizes a pupil filterplaced on the projection pupil plane 514. By way of example, the pupilfilter may be employed to filter out specific spatial frequencycomponents of the EUV light reflected from the mask 508. In someembodiments, the pupil filter may serve as a phase pupil filter thatmodulates the phase distribution of the light directed through theprojection optics 510.

As discussed above, the lithography system 500 also includes thesubstrate stage 518 to secure the semiconductor substrate 516 to bepatterned. In various embodiments, the semiconductor substrate 516includes a semiconductor wafer, such as a silicon wafer, germaniumwafer, silicon-germanium wafer, III-V wafer, or other type of wafer asdescribed above or as known in the art. The semiconductor substrate 516may be coated with a resist layer (e.g., an EUV resist layer) sensitiveto EUV light. EUV resists may have stringent performance standards. Inthe embodiments described herein, the various subsystems of thelithography system 500, including those described above, are integratedand are operable to perform lithography exposing processes including EUVlithography processes. The lithography system 500 may further includeother modules or subsystems which may be integrated with (or be coupledto) one or more of the subsystems or components described herein.

The EUV light system 100 (FIG. 1) may be used as the source 502 orprovide the EUV radiation to the source 502 for use by the lithographysystem 500. That is, the system 100 provides the EUV radiation at whichpoint it is transferred to the systems described in the lithographysystem 500.

FIG. 6 is a flow chart of a method 600 for residual gain monitoring in alaser system according to various aspects of the present disclosure. Themethod 600 is merely an example, and is not intended to limit thepresent disclosure beyond what is explicitly recited in the claims.Additional steps can be provided before, during, and after the method600, and some of the steps described can be replaced, relocated, oreliminated for other embodiments of the method 600. The method 600 isdescribed below in conjunction with FIGS. 3A-3B.

At operation 602, the method 600 generates a laser beam 114 (FIG. 3A) ina laser source 118. The laser beam 114 may be a series of pulses. Atoperation 604, the method 600 passes the laser beam 114 through a laseramplifier 120 along a main beam path. At operation 606, the method 600generates a probe laser beam 166 in a residual gain monitor 160. Theprobe laser beam 166 has a pulse energy and average power level muchless than the corresponding amplifier saturation energy and power. Theprobe laser beam 166 has a power level much less than the laser beam114. At operation 608, the method 600 passes the probe laser beamthrough the laser amplifier 120 along a probe beam path, wherein theprobe beam path can be switched by rotating the probe beam inputreflectors to become offset from the main beam path (FIG. 3A) ordiagonally overlaps with the main beam path (FIG. 3B). At operation 610,the method 600 calculate a residual gain of the laser amplifier based onthe strengths of the probe laser beam 166 before and after beingamplified by the laser amplifier. The operation 610 may further includesending the residual gain information to a control module 170 foradjusting parameters of the laser system. The method 600 may optionallyhave an operation 612. The operation 612 includes adjusting the timedelay between the laser pulse and probe pulse, for example through theprobe beam delay line 183 as illustrated in FIG. 4D. The temporalevolution of the amplifier gain depletion can therefore be furtherresolved through tomography to finely optimize the laser dynamics in thesystem.

The various embodiments described herein offer several advantages overthe existing art. It will be understood that not all advantages havebeen necessarily discussed herein, no particular advantage is requiredfor all embodiments, and other embodiments may offer differentadvantages. For example, embodiments discussed herein provide ametrology apparatus and methods thereof for residual gain monitoring andreduction in a laser system, which reduces heat dissipation and opticalcomponent damages in a EUV lithography production flow. Furthermore,various embodiments of the present disclosure can be implemented withlow complexity and low manufacturing cost.

In one exemplary aspect, the present disclosure is directed to a system.The system includes a laser source operable to provide a laser beam; alaser amplifier having an input port and an output port and operable toamplify the laser beam, the laser beam travelling along a main beam paththrough the laser amplifier from the input port to the output port; anda residual gain monitor operable to provide a probe laser beam, theprobe laser beam travelling along a probe beam path through the laseramplifier from the output port to the input port and returning to theresidual gain monitor, wherein the residual gain monitor is operable tocalculate a residual gain of the laser amplifier using the probe laserbeam. In an embodiment, the system further includes an extremeultraviolet (EUV) vessel operable to receive the laser beam aftertravelling through the laser amplifier for interaction with a target tocreate EUV light. In an embodiment, the system further includes a probelaser beam delay line, wherein the probe laser beam delay line isconfigured to adjust a time interval between when the laser beam and theprobe laser beam entering the laser amplifier. In an embodiment, theprobe beam path is offset from the main beam path in the laseramplifier; and the probe laser beam travels through the laser amplifierin parallel with the laser beam. In an embodiment, the probe beam pathoverlaps with the main beam path in the laser amplifier in a diagonaldirection; and the probe laser beam travels through the laser amplifierduring a period when there is no laser beam in the laser amplifier. Inan embodiment, a portion of the laser beam is reflected after havingpassed the laser amplifier, resulting in a reflected laser beam; and theprobe laser beam travels through the laser amplifier before thereflected laser beam enters the laser amplifier. In an embodiment, apower level of the probe laser beam before the probe laser beam entersthe laser amplifier is in a range from about 10⁻⁹ to about 10⁻⁶ of apower level of the laser beam before the laser beam enters the laseramplifier. In an embodiment, the system further includes a reflectingcomponent allowing the laser beam to travel through the laser amplifiermore than once. In an embodiment, the reflecting component also allowsthe probe laser beam to travel through the laser amplifier more thanonce. In an embodiment, wherein the reflecting component changes apolarization direction of the laser beam by 90°.

In another exemplary aspect, the present disclosure is directed to asystem. The system includes a laser source generating a laser pulse, thelaser pulse traveling along a laser path; an extreme ultraviolet (EUV)vessel receiving the laser pulse for creating EUV light and reflecting aportion of the laser pulse as a reflected laser pulse; a first gainmedium located between the laser source and the EUV vessel along thelaser path; a second gain medium located between the first gain mediumand the EUV vessel along the laser path, wherein the reflected laserpulse travels through the second gain medium and the first gain mediumalong the laser path; a first residual gain monitoring module generatinga first probe laser pulse, the first probe laser pulse traveling throughthe first gain medium along a first probe path, the first residual gainmonitoring module generating a first residual gain data according to thefirst probe laser pulse; a second residual gain monitoring modulegenerating a second probe laser pulse, the second probe laser pulsetraveling through the second gain medium along a second probe path, thesecond residual gain monitoring module generating a second residual gaindata according to the second probe laser pulse; and a control modulecoupled to the first and second residual gain monitoring modules toreceive the first and second residual gain data and adjusting parametersof the first and second gain mediums accordingly. In an embodiment, thefirst probe path overlaps with the laser path in the first gain medium;and the second probe path overlaps with the laser path in the secondgain medium. In an embodiment, the first probe laser pulse travelsthrough the first gain medium during an interval after the laser pulseleaves the first gain medium and before the reflected laser pulse entersthe first gain medium; and the second probe pulse travels through thesecond gain medium during an interval after the laser pulse leaves thesecond gain medium and before the reflected laser pulse enters thesecond gain medium. In an embodiment, the first probe path is offsetfrom the laser path in the first gain medium; and the second probe pathis offset from the laser path in the second gain medium. In anembodiment, wherein the second gain medium has a cross-sectional areaperpendicular to the laser path larger than that of the first gainmedium, the system further includes a beam shaping component locatedbetween the first gain medium and the second gain medium, the beamshaping component enlarging a cross-sectional area of the laser pulse.In an embodiment, the laser pulse travels through the first gain mediummore than once before entering the second gain medium. In an embodiment,parameters of the first and second gain mediums include RF power,mixture gas pressure, mixture gas ratio, or a combination thereof.

In yet another exemplary aspect, the present disclosure is directed to amethod. The method includes generating a laser beam in a laser source;passing the laser beam through a laser amplifier along a main beam path,such that the laser beam is amplified, the laser beam exiting the laseramplifier from a terminal of the laser amplifier; generating a probelaser beam in a residual gain monitor; passing the probe laser beamthrough the laser amplifier along a probe beam path, such that the probelaser beam is amplified, the probe laser beam entering the laseramplifier from the terminal of the laser amplifier; and calculating aresidual gain of the laser amplifier based on strengths of the probelaser beam before and after being amplified. In an embodiment, the probebeam path is offset from the main beam path in the laser amplifier, thepassing of the probe laser beam through the laser amplifier includespassing the probe laser beam through the laser amplifier simultaneouslywith the passing of the laser beam through the laser amplifier. In anembodiment, the probe beam path overlaps with the main beam path in thelaser amplifier, the passing of the probe laser beam through the laseramplifier includes passing the probe laser beam through the laseramplifier during a period when the laser beam is not in the laseramplifier.

In yet another exemplary aspect, the present disclosure is directed to alaser system. The laser system includes a laser source configured togenerate a first laser beam; a laser amplifier configured to amplify thefirst laser beam; a reflector configured to reflect the first laser beamback to the laser amplifier to become a second laser beam, whereinpolarization directions of the first and second laser beams differ by90; and a polarization beam splitter located between the laser sourceand the laser amplifier, the polarization beam splitter passing thefirst laser beam and reflecting the second laser beam. In an embodiment,the laser system further includes an extreme ultraviolet (EUV) vesselconfigured to receive the second laser beam reflected from thepolarization beam splitter, wherein a portion of the second laser beamis reflected from a droplet in the EUV vessel to become a third laserbeam, the third laser beam traveling back to the polarization beamsplitter, wherein the polarization beam splitter substantially fullypasses the third laser beam. In an embodiment, the first laser beampasses the polarization beam splitter in a first direction; and thethird laser beam passes the polarization beam splitter in a seconddirection perpendicular to the first direction. In an embodiment, aresidual gain monitor configured to generate a probe laser beam, theprobe laser beam traveling through the laser amplifier. In anembodiment, the residual gain monitor calculates a residual gain of thelaser amplifier according to strengths of the probe laser beam beforeand after traveling through the laser amplifier.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A system, comprising: a laser source operable to provide a laser beam; a laser amplifier having an input port and an output port and operable to amplify the laser beam, the laser beam travelling along a main beam path through the laser amplifier from the input port to the output port; and a residual gain monitor operable to provide a probe laser beam, the probe laser beam travelling along a probe beam path through the laser amplifier from the output port to the input port and returning to the residual gain monitor, wherein the residual gain monitor is operable to calculate a residual gain of the laser amplifier using the probe laser beam.
 2. The system of claim 1, further comprising: an extreme ultraviolet (EUV) vessel operable to receive the laser beam after travelling through the laser amplifier for interaction with a target to create EUV light.
 3. The system of claim 1, further comprising a probe laser beam delay line, wherein the probe laser beam delay line is configured to adjust a time interval between when the laser beam and the probe laser beam entering the laser amplifier.
 4. The system of claim 1, wherein: the probe beam path is offset from the main beam path in the laser amplifier; and the probe laser beam travels through the laser amplifier in parallel with the laser beam.
 5. The system of claim 1, wherein: the probe beam path overlaps with the main beam path in the laser amplifier in a diagonal direction; and the probe laser beam travels through the laser amplifier during a period when there is no laser beam in the laser amplifier.
 6. The system of claim 5, wherein: a portion of the laser beam is reflected after having passed the laser amplifier, resulting in a reflected laser beam; and the probe laser beam travels through the laser amplifier before the reflected laser beam enters the laser amplifier.
 7. The system of claim 1, wherein a power level of the probe laser beam before the probe laser beam enters the laser amplifier is in a range from about 10⁻⁹ to about 10⁻⁶ of a power level of the laser beam before the laser beam enters the laser amplifier.
 8. The system of claim 1, further comprising: a reflecting component allowing the laser beam to travel through the laser amplifier more than once.
 9. The system of claim 8, wherein the reflecting component also allows the probe laser beam to travel through the laser amplifier more than once.
 10. The system of claim 8, wherein the reflecting component changes a polarization direction of the laser beam by 90°.
 11. A system, comprising: a laser source generating a laser pulse, the laser pulse traveling along a laser path; an extreme ultraviolet (EUV) vessel receiving the laser pulse for creating EUV light and reflecting a portion of the laser pulse as a reflected laser pulse; a first gain medium located between the laser source and the EUV vessel along the laser path; a second gain medium located between the first gain medium and the EUV vessel along the laser path, wherein the reflected laser pulse travels through the second gain medium and the first gain medium along the laser path; a first residual gain monitoring module generating a first probe laser pulse, the first probe laser pulse traveling through the first gain medium along a first probe path, the first residual gain monitoring module generating a first residual gain data according to the first probe laser pulse; a second residual gain monitoring module generating a second probe laser pulse, the second probe laser pulse traveling through the second gain medium along a second probe path, the second residual gain monitoring module generating a second residual gain data according to the second probe laser pulse; and a control module coupled to the first and second residual gain monitoring modules to receive the first and second residual gain data and adjusting parameters of the first and second gain mediums accordingly.
 12. The system of claim 11, wherein: the first probe path overlaps with the laser path in the first gain medium; and the second probe path overlaps with the laser path in the second gain medium.
 13. The system of claim 12, wherein: the first probe laser pulse travels through the first gain medium during an interval after the laser pulse leaves the first gain medium and before the reflected laser pulse enters the first gain medium; and the second probe pulse travels through the second gain medium during an interval after the laser pulse leaves the second gain medium and before the reflected laser pulse enters the second gain medium.
 14. The system of claim 11, wherein: the first probe path is offset from the laser path in the first gain medium; and the second probe path is offset from the laser path in the second gain medium.
 15. The system of claim 11, wherein the second gain medium has a cross-sectional area perpendicular to the laser path larger than that of the first gain medium, further comprising: a beam shaping component located between the first gain medium and the second gain medium, the beam shaping component enlarging a cross-sectional area of the laser pulse.
 16. The system of claim 11, wherein the laser pulse travels through the first gain medium more than once before entering the second gain medium.
 17. The system of claim 11, wherein parameters of the first and second gain mediums include RF power, mixture gas pressure, mixture gas ratio, or a combination thereof.
 18. A method, comprising: generating a laser beam in a laser source; passing the laser beam through a laser amplifier along a main beam path, such that the laser beam is amplified, the laser beam exiting the laser amplifier from a terminal of the laser amplifier; generating a probe laser beam in a residual gain monitor; passing the probe laser beam through the laser amplifier along a probe beam path, such that the probe laser beam is amplified, the probe laser beam entering the laser amplifier from the terminal of the laser amplifier; and calculating a residual gain of the laser amplifier based on strengths of the probe laser beam before and after being amplified.
 19. The method of claim 18, wherein the probe beam path is offset from the main beam path in the laser amplifier, the passing of the probe laser beam through the laser amplifier includes passing the probe laser beam through the laser amplifier simultaneously with the passing of the laser beam through the laser amplifier.
 20. The method of claim 18, wherein the probe beam path overlaps with the main beam path in the laser amplifier, the passing of the probe laser beam through the laser amplifier includes passing the probe laser beam through the laser amplifier during a period when the laser beam is not in the laser amplifier. 